1. Field of the Invention
The present invention relates to an afocal wide field of view optical system and more particularly to a system having four non-spherical mirrors.
2. Description of Related Art
Wide field of view optical systems, such as reflective, afocal telescopes are utilized in multispectral applications and in certain space sensor systems. The use of these wide field of view optical systems is to provide larger unobscured two dimensional field of view ranges by the system. The image space field of view may be viewed at the exit pupil by various types of sensors which consist of an imaging optical system and a detector array. This information may be separated by dichroic beamsplitters and used by several sensors or refractive imaging optics with detector arrays operating in different spectral bands of interest. The most familiar type of sensor would be the human eye which has a single lens imaging system and a light sensitive retina, which translates light energy into nervous energy.
Prior wide field of view reflective optical systems include afocal telescopes having two confocal paraboloidal mirrors and three mirror anastigmat design. Both types of telescopes have several disadvantages. The two mirror design suffers from field curvature aberrations. As the magnification ratios increase in the systems from 1.3.times. to 4.0.times., the curvature of the secondary mirror increases thus increasing the telescopes field curvature aberrations. These uncorrected field curvature aberrations severely limit the usefulness of these two mirror designs beyond about 3 degrees at a 4.times. magnification ratio. The three mirror design provides a useable object space field of view coverage of approximately 5 degrees circular field of view beyond which field aberrations, such as coma and astigmatism, severely degrade optical performance. Thus, there is a need in the art to provide an all reflective telescope having a wide field of view (greater than 5 degrees, circular) at a 4.times. magnification which enables multi-spectral operation with sensors or refractive imagers to be achieved.